Light of wavelength 193 nm or longer has hitherto been used as the exposure light in lithographic equipment used when manufacturing semiconductor devices such as integrated circuits, liquid crystal displays, and thin film magnetic heads. The surfaces of lenses used in projection optical systems of such lithographic equipment are normally spherical, and the accuracy in the lens shape is 1 to 2 nm RMS (root mean square).
With the advance in microminiaturization of the patterns on semiconductor devices in recent years, there has been a demand for exposure apparatus that use wavelengths shorter than those used heretofore to achieve even greater microminiaturization. In particular, there has been a demand for the development and manufacture of projection exposure apparatus that use soft X-rays of wavelength of 11 to 13 nm.
Lenses (i.e., dioptric optical elements) cannot be used in the EUV wavelength region due to absorption, so catoptric projection optical systems (i.e., systems comprising only reflective surfaces) are employed. In addition, since a reflectance of only about 70% can be expected from reflective surfaces in the soft X-ray wavelength region, only three to six reflective surfaces can be used in a practical projection optical system.
Accordingly, to make an EUV projection optical system aberration-free with just a few reflective surfaces, all reflective surfaces are made aspheric. Furthermore, in the case of a projection optical system having four reflective surfaces, a reflective surface shape accuracy of 0.23 nm RMS is required. One method of forming an aspheric surface shape with this accuracy is to measure the actual surface shape using an interferometer and to use a corrective grinding machine to grind the surface to the desired shape.
In a conventional surface-shape-measuring interferometer, measurement repeatability is accurate to 0.3 nm RMS, the absolute accuracy for a spherical surface is 1 nm RMS, and the absolute accuracy of an aspheric surface is approximately 10 nm RMS. Therefore, the required accuracy cannot possibly be satisfied. As a result, a projection optical system designed to have a desired performance cannot be manufactured.
So-called null interferometric measurement using a null (compensating) element has hitherto been conducted for the measurement of aspheric surface shapes. Null lenses that use spherical lenses comprising spherical surfaces, and zone plates wherein annular diffraction gratings are formed on plane plates have principally been used as null elements.
FIG. 1 shows a conventional interferometer system 122 arrangement for null measurement using a null (compensation) element 132. The interferometric measurement described herein is a slightly modified version of a Fizeau interferometric measurement. Namely, a plane wave 126 emitted from an interferometric light source 124 is partially reflected by a high-precision Fizeau surface 130 formed on a Fizeau plane plate 128. The component of plane wave 126 transmitted through Fizeau surface 130 is converted into measurement wavefront (null wavefront) 134 by null element 132 and assumes a desired aspheric design shape at a measurement reference position RP, following which it arrives at a test surface 138 of a test object 136 previously set at the reference position. The light arriving at test surface 138 is reflected therefrom and interferes with the light component reflected from Fizeau surface 130, and forms monochromatic interference fringes inside interferometer system 122. These interference fringes are detected by a detector such as a CCD (not shown). A signal outputted by the detector is analyzed by an information processing system (not shown) that processes the interferometer information contained in the output signal. Similar measurements can be performed using a Twyman-Green interferometer. To accurately ascertain the shape of test surface 138, the null element 132 must be manufactured with advanced technology, since there must be no error in the null wavefront. Specifically, this means that the optical characteristics of the null element 132 must be measured beforehand with high precision. Based on these measurements, the shape of null wavefront 134 is then determined by ray tracing. This results in the manufacture of null element 132 taking a long time. Consequently, the measurement of the desired aspheric surface takes a long time.
FIG. 2 shows another example of a conventional Fizeau interferometer 222. Referring to FIG. 2, laser light from laser 224 passes through a lens system 226 to become a collimated light beam of a prescribed diameter and is incident Fizeau plate 228. Rear side 230 of Fizeau plate 228 is accurately ground to a highly flat surface, and the component of the incident light reflected by rear side 230 of Fizeau plate 228 becomes a reference beam having a plane wavefront. The component of incident light transmitted through a Fizeau plate 228 passes through null element 232, where the plane wavefront where the plane wavefront is converted to a desired aspheric wavefront. The aspheric wavefront is then incident in perpendicular fashion an aspheric test surface 238. The light reflected by test surface 238 returns along the original optical path, is superimposed on the reference light beam, reflects off a beam splitting element 256 in lens system 226, and forms interference fringes on a CCD detector 260. By processing these interference fringes by a computer (not shown), the shape error can be measured.
A problem with interferometer 222 is deterioration, in absolute accuracy, due to null element 232. A null element comprising a number of high-precision lenses (e.g., lenses 234 and 236) a CGH (computer-generated hologram), or the like is ordinarily used as null element 232, and manufacturing errors on the order of 10 nm RMS typically result.
Since interferometer 222 tends to be affected by vibration and air fluctuations due to the separation of reference surface 230 (i.e., rear side of Fizeau plate 228) and test surface 238. Repeatability is also poor, at 0.3 nm RMS. Furthermore, in measuring an aspheric surface, alignment of null element 232 and test surface 238 is critical. Measurement repeatability deteriorates by several nanometers if alignment accuracy is poor.